 Characteristic impedance silicon transistor RF Cafe Forums 

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darcyrandall2004 Post subject: Characteristic impedance silicon transistor Posted: Tue Feb 27, 2007 11:19 pm
Colonel
Joined: Tue Feb 27, 2007 6:16 am Posts: 46 I am designing a UHF transmitter for the sake of learning an interest.
My question is: Do the surface mount components i.e transistors, I place on the PCB, take on the characteristic impedance of the PCB or do they have a characteristic impedance of 50 ohm?
For example, the datasheet for the BFR182 surface mnt transistor provides S parameters and indicates that the S parameters provided were measured with a characteristic impedance, Zo of 50ohm.
The PCB I will be using I calculate to have a 116ohm Zo.
It is not practical to design a PCB with a Zo of 50ohm.
Do I calculate the impedances of my transistor using the Zo of 50ohm, or do I calculate the impedances of the transistor using the PCB Zo of 116 ohm? Thankyou
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fred47 Post subject: S ParametersPosted: Wed Feb 28, 2007 3:27 am
General
Joined: Wed Feb 22, 2006 3:51 pm Posts: 104 Hi! You seem to be missing the relationship between impedance and S parameters.
So, a quick review: to measure impedance, you apply a stimulus (a voltage or current) and measure the resulting value (a current, if a voltage is applied, or a voltage if a current is applied).
That's how DC Ohmmeters work, but it gets difficult at VHF and higher frequencies because we can neither 1. generate an approximatelyideal voltage or current, nor 2. measure a current or voltage without interaction.
The problem is stray capacitance and stray inductance. For example, a mere 1 pF in parallel at 1 GHz can cause huge errors in measuring impedances.
So if you can't measure voltages or currents accurately, what can you measure? The short answer is power. A slightly longer answer is "reflected or forward power". You do this using transmission lines rather than plain wires, of course, and it turns out that we can measure both the power and the angle it's at.
For this measurement to be exact, you need to know the characteristic impedance of the transmission line. If you use coaxial cable, this is usually either 50 or 75 Ohms. If you're using microstrip on PCB material, the characteristic impedance is controllable by the designer, by selecting the width of the line and the thickness of the material.
Of course, for a 50 Ohm system, you need a signal generator with an accurate 50 Ohm output impedance (for the stimulus), and an accurate 50 Ohm load. And you'll also need one or more directional couplers, and some way of measuring power.
So suppose that we have a 50 Ohm generator, connected to 50 Ohm coax, connected to a 75 Ohm resistor, connected to a 50 Ohm load. The impedance at the resistor will be 75 Ohms in parallel with 50 Ohms, so where the resistor is connected the impedance will be less than 50 Ohms.
Where the impedance changes, power is reflected. You can measure this power with a directional coupler. An Agilent 8510 Vector Network Analyzer ($$$) is a couple of glorified directional couplers, with a source and power meter.
What the input S parameter is in a 50 Ohm system, is the reflection coefficient (expressed as a magnitude and a phase) seen by a 50Ohm line connected to the transistor input.
You can calculate the actual input impedance if you know S11 and Z0, the system's characteristic impedance. I posted the formulas in two recent threads here on RF CafeCircuits Forum, the 50to75 Ohm thread, and the amplifier circuit design thread  I won't repeat them here unless asked.
You make a couple of justsimplywrong statements in your posting. Here are some related facts:
1. Designers use 50 Ohm microstrip (=PCB traces) every day of the year  it's not hard at all. For a calculator, you can go to www.ultracad.com or check any number of books.
2. PCB's do not have an impedance  the impedance is a function of the line width and board thickness over the ground plane. People try to restrict the range to about 25100 Ohms, but you can get higher or lower values.
Transistors NEITHER take on the characteristic impedance of the circuit they're used with, nor do they themselves have a characteristic impedance. The input and output impedances, and the forward and reverse gains, are all functions of the operating voltages and currents.
I hope this helps.
Good Luck, Fred
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darcyrandall2004 Post subject: Posted: Wed Feb 28, 2007 9:53 am
Colonel
Joined: Tue Feb 27, 2007 6:16 am Posts: 46 Hello Fred.
So to determine what my input and output impedances of components are, I simply base my calculation on the given S parameters, input/output reflection coeffs and convert this to Z parameters and then multiply by the Zo of 50 given in the datasheets?
What about the transistors legs? Surely the S parameters of the transistor would change depending on the characteristic impedance of the PCB it was placed on and the length of the transistors legs?
Ive been using http://www.emclab.umr.edu/pcbtlc/microstrip.html to calculate my characteristic impedance. The values used are
ER 4.2 20 mil track width. 63mil or 1.6mm board thickness. 1oz copper Zo = 113ohms
If I want a 50ohm characteristic impedance on this board(pref.) my track width would have to be 120mil or otherwise use a 30mil thick board and 50mil wide tracks. Again this track width is too large.
What values do more experienced designers use? Different materials for the pcb perhaps?
Your help is very much appreciated. Cheers
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fred47 Post subject: Transistor amplifiersPosted: Wed Feb 28, 2007 1:21 pm
General
Joined: Wed Feb 22, 2006 3:51 pm Posts: 104 Hi Darcy!
Good choice for the microstrip calculator! The folks at UMRolla are pretty sharp, and very helpful.
I'm a bit puzzled, though  on what grounds is a 50 mil trace "too wide"?
You have two choices for impedance matching: 1. lumped components (inductors and capacitors), and 2. transmission lines.
If you're in the frequency range where you can use lumped components (and with modern SMT parts, that range is moving upward!), not all traces need to be 50 Ohm microstrip. One "rule of thumb" is that a segment less than 1/10 of a wavelength won't affect things too much. That's one advantage of computer programs  you can include the interconnections as circuit elements when you do your analysis. (Still, with modern transistors, 1/10 wavelength may be at a much higher frequency than your chosen operating frequency, so care is necessary!)
As you surmise, designers often use other materials  if you read the literature and the online material, you'll find lots of references to materials made by Rogers  ROxxxx, where xxxx is a number.
Good Luck! Fred
Posted 11/12/2012



